linux-kernel - Re: [GIT PULL] KVM fixes for 3.5-rc6

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Date:	Fri, 13 Jul 2012 21:02:20 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Avi Kivity <avi@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [GIT PULL] KVM fixes for 3.5-rc6

On Fri, 13 Jul 2012, Linus Torvalds wrote:

> On Fri, Jul 13, 2012 at 11:28 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > We already discussed to let the irq chip (in this case MSI) tell the
> > core that it does not need the extra oneshot handling. That way the
> > code which requests an threaded irq with the NULL primary handler
> > works on both MSI and normal interrupts.
> 
> So I  don't think your patch is quite right.
> 
> If you want to clear the IRQF_ONESHOT for MSI irq's (and other ones
> where the interrupt controller is fundamentally ONESHOT), I think you
> should do it a few lines higher up - *before* you check the "does the
> IRQF_ONESHOT mask match other shared interrupts"?
> 
> Now, irq sharing presumably doesn't happen with MSI, but there's
> nothing fundamentally wrong with message-based irq schemes that have
> shared interrupt handlers.
> 
> I think. Hmm?

Shared irqs are not supported by MSI, but yes, the check should be
done way up. Makes it less ugly as well :)

Thanks,

	tglx

Index: linux-2.6/arch/x86/kernel/apic/io_apic.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/apic/io_apic.c
+++ linux-2.6/arch/x86/kernel/apic/io_apic.c
@@ -3109,6 +3109,7 @@ static struct irq_chip msi_chip = {
 	.irq_set_affinity	= msi_set_affinity,
 #endif
 	.irq_retrigger		= ioapic_retrigger_irq,
+	.flags			= IRQCHIP_ONESHOT_SAFE,
 };
 
 static int setup_msi_irq(struct pci_dev *dev, struct msi_desc *msidesc, int irq)
Index: linux-2.6/include/linux/irq.h
===================================================================
--- linux-2.6.orig/include/linux/irq.h
+++ linux-2.6/include/linux/irq.h
@@ -351,6 +351,7 @@ enum {
 	IRQCHIP_MASK_ON_SUSPEND		= (1 <<  2),
 	IRQCHIP_ONOFFLINE_ENABLED	= (1 <<  3),
 	IRQCHIP_SKIP_SET_WAKE		= (1 <<  4),
+	IRQCHIP_ONESHOT_SAFE		= (1 <<  5),
 };
 
 /* This include will go away once we isolated irq_desc usage to core code */
Index: linux-2.6/kernel/irq/manage.c
===================================================================
--- linux-2.6.orig/kernel/irq/manage.c
+++ linux-2.6/kernel/irq/manage.c
@@ -960,6 +960,18 @@ __setup_irq(unsigned int irq, struct irq
 	}
 
 	/*
+	 * Drivers are often written to work w/o knowledge about the
+	 * underlying irq chip implementation, so a request for a
+	 * threaded irq without a primary hard irq context handler
+	 * requires the ONESHOT flag to be set. Some irq chips like
+	 * MSI based interrupts are per se one shot safe. Check the
+	 * chip flags, so we can avoid the unmask dance at the end of
+	 * the threaded handler for those.
+	 */
+	if (desc->irq_data.chip->flags & IRQCHIP_ONESHOT_SAFE)
+		new->flags &= ~IRQF_ONESHOT;
+
+	/*
 	 * The following block of code has to be executed atomically
 	 */
 	raw_spin_lock_irqsave(&desc->lock, flags);
@@ -1033,7 +1045,8 @@ __setup_irq(unsigned int irq, struct irq
 		 */
 		new->thread_mask = 1 << ffz(thread_mask);
 
-	} else if (new->handler == irq_default_primary_handler) {
+	} else if (new->handler == irq_default_primary_handler &&
+		   !(desc->irq_data.chip->flags & IRQCHIP_ONESHOT_SAFE)) {
 		/*
 		 * The interrupt was requested with handler = NULL, so
 		 * we use the default primary handler for it. But it


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